Voltage-controlled-oscillators (VCOs) are often used as the frequency source for phase-locked-loops (PLLs) when implementing frequency synthesizers. However, it is difficult to make the PLLs stable over a wide tuning range (e.g. from 500 MHz to 1 GHz) for locking onto external signals. Most of the dynamic range on the loop capacitor (for supplying the voltage input to the VCO) is wasted because of the process and temperature variation of the CMOS process. The voltage required by the loop capacitor can go to very high levels under certain process and temperatures such as those in the C11 (0.12 micrometer) and L90 (0.09 micrometer) processes, which can cause severe leakage problems.
It would be desirable to have a tunable oscillator such as a VCO which has a relatively constant gain over a broad frequency range and is immune to power supply, process and temperature variation.